Low-loss, high-isolation, fiber-optic isolator

Abstract

A low-loss, high-isolation, fiber-optic isolator for use in single-mode fiber systems utilizes a Faraday rotator and two polarizers, one at each end angularly oriented from each other at the angle of rotation for isolation, and two aspheric lens connectors to couple optical fibers to the Faraday isolator to reduce forward loss to about 2.6 dB and improve isolation to greater than 70 db.

I claim: 
     
       1. A low-loss, high-isolation apparatus for isolating a source of an incident light beam transmitted through a first optical fiber a first portion of said beam of light reflected by a second optical fiber and scattered by an isolator assembly and for transmitting a second portion of said beam of light to said second optical fiber from said first optical fiber, comprising an isolator assembly comprised of a Faraday rotator with parallel planar surfaces on opposite sides and two polarizers with planar surfaces parallel to said planar surfaces of said rotators, said polarizers being oriented at an angle between their directions of polarization equal to the angle α of rotation of said Faraday rotator, and   two aspheric lenses having their aspheric surfaces facing each other, and having said isolator assembly between them, and each having planar surfaces opposite said aspheric surfaces parallel to said planar surfaces of said rotator, each of said aspheric lenses having their axes angularly and laterally aligned, and having a narrow angle of acceptance of light, said narrow angle of acceptance being less than about 0.003 radians offset from the axes of said aspheric lenses for a loss of -30 dB, and less than about 0.0045 radians from their axes for a loss of -70 dB.   
     
     
       2. Apparatus as defined in claim 1 wherein said first optical fiber is a single-mode optical fiber connected to said planar surface of an aspheric lens on the input side of said assembly, and said second optical fiber is a single-mode optical fiber connected to said planar surface of an aspheric lens on the exit side of said assembly. 
     
     
       3. A low-loss, high-isolation apparatus of isolating a beam of light transmitted through a first optical fiber a first portion of said beam of light reflected by a second optical fiber and scattered in an isolator assembly and for transmitting a second portion of said beam of light to said second optical fiber, comprising an isolator assembly for coupling said first optical fiber to said second optical fiber, said isolator assembly having a Faraday rotator and two polarizers, said polarizers being oriented at an angle between their directions of polarization equal to an angle α of rotation of said Faraday rotator, and   two aspheric lenses on opposite ends of said isolator assembly oriented with their aspheric surfaces facing each other and their axes substantially aligned angularly and laterally, each aspheric lens having a narrow angle of acceptance of light through said aspheric surface, said narrow angle of acceptance being less than about 0.0045 radians offset from the axis of said aspheric lens for a loss of -70 dB,   each aspheric lens having a planar surface opposite said aspheric surface and perpendicular to the axis of said aspheric lens for interfacing with a separate one of said first and second optical fibers.   
     
     
       4. A low-loss, high-isolation apparatus as defined in claim 3 wherein said narrow angle of acceptance is about 0.003 radians offset from the axis of said aspheric lens for a loss of -30 dB. 
     
     
       5. A low-loss, high-isolation, fiber-optic coupler for isolating a beam of light transmitted through a first optical fiber to a second optical fiber from light reflected by said second optical fiber and scattered in said coupler comprising an optical isolator consisting of a Faraday rotator having parallel faces and two polarizers, one on each side of said Faraday rotator position between said first and second optical fibers, said polarizers having parallel interfaces with said parallel faces of said Faraday rotator, each of said polarizers further having an outer face parallel to its interface with said Faraday rotator, said two polarizers being oriented relative to each other at an angle between their directions of polarization equal to an angle α of rotation of said Faraday rotator,   means for coupling light transmitted through said first optical fiber into said optical isolator comprising an aspheric lens having its axis aligned with the axis of said first optical fiber and normal to the outer face of the adjacent one of said two polarizers, said aspheric lens having a planar surface interfacing with an end face of said first optical fiber, and an aspheric surface facing the adjacent one of said polarizers, said aspheric surface being shaped to accept light from said adjacent polarizer and coupling light accepted into the end face of said first optical fiber within a limited angle from said axis,   means for coupling light from said second optical fiber into said optical isolator comprising an aspheric lens having its axis aligned with the axis of said second optical fiber and normal to the outer face of the adjacent one of said two polarizers, said aspheric lens having a planar surface interfacing with an end face of said second optical fiber, and an aspheric surface facing the adjacent one of said two polarizers, said aspheric surface being shaped for acceptance of light from said adjacent polarizers and coupling light accepted into said second optical fiber within a limited angle from said axis.